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A firewall guarantees the security of the internet.
Literal
Firewall [subject-が] internet [genitive-の] security [object-を] guarantees.
A textbook-style IT definition — notable not for the grammar but for how much it oversimplifies the actual claim. A firewall contributes to security rather than 'guaranteeing' it, so this is either marketing boilerplate or a simplistic explanation written for non-technical readers. Structurally it's minimum-viable Japanese: SOV with a subject (ファイアーウォール), an object (安全性), and a transitive verb (保証する 'guarantee'). 安全性 uses the productive ~性 noun suffix to mean 'security, safeness' (parallel to 危険性 'dangerousness' or 発がん性 'carcinogenicity'). 保証する ('to guarantee, to assure') is a formal verb from business and legal register.